Anion receptors are used as additives to non-aqueous electrolytes in lithium ion batteries to improve conductivity. Past research has been carried out on reducing the ion-pairing in non-aqueous electrolytes to enhance the conductivity of such solutions. The goal was to find ligands that could coordinate with either cations or anions, and hence increase the cation-anion distance and decrease the ion-pairing. Research on this issue has been summarized in a review article by F. P. Schidtchen et al., Chem. Rev., 97, 1609 (1997).
Recent research has focused on providing neutral compounds as anion receptors for non-aqueous electrolytes. More specifically, in the field of lithium-ion batteries, an anion receptor is expected to form a complex with the anion of a lithium salt, and hence increase the transference number of lithium ions, to improve the conductivity of the electrolyte. A number of patents have disclosed various boron compounds suitable for this use, including U.S. Pat. No. 5,849,432 to Angell et al., and U.S. Pat. Nos. 6,352,198 and 6,022,643 to Lee et al. To improve the conductivity of the non-aqueous electrolyte, anion receptors are typically used at concentrations much greater than 0.1 molar (M), such as 0.5-1.0 M, so that they essentially function as a co-solvent to dissolve LiF and form new LiF salt-based electrolytes.
While previous work has demonstrated the capability of anion receptors to improve the conductivity of non-aqueous battery electrolytes, drawbacks to the use of anion receptors have not been widely recognized. As demonstrated herein, the conductivity gain of the electrolyte can be quickly offset by the impedance increase over the passivation films if high concentrations of anion receptors are employed. For example, a lithium-ion cell with 5 weight percent (wt %) tris(pentafluorophenyl)borane in the electrolyte (corresponding to a concentration of 0.1 M), has a higher overall impedance than the same cell without the additive. Hence, even though high concentrations of anion receptors improve the conductivity by dissolving LiF, impedance is also increased, leading to inferior cell performance as compared to electrolytes that lack anion receptors. Accordingly, new electrolyte solutions that retain good conductivity and low impedance are highly desirable.